A new strain of the MRSA "superbug" has been found in British cows and is believed to be infecting humans. Reaserchers found that, like other MRSA strains, it was resistant to the most commonly used antibiotics. However, the bug was found to be genetically very different. Subsequent research showed that the strain was also present in humans. Dr Garcia-Alvarez says that finding a new strain in both in humans and cows is "very worrying".

Environmental campaigners say the new strain has emerged because of the over-use of antibiotics by dairy farmers. Antibiotics are widely used by dairy farmers to treat cows with mastitis. However over-use means some bacteria become resistant and difficult to treat if humans become infected.

Dr Mark Holmes of Cambridge University, who led the research, said this was a "credible hypothesis".

The researchers, writing in the Lancet Infectious Diseases Journal, say there is no additional health risk from eating milk and dairy products.

Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium responsible for several difficult-to-treat infections in humans. It may also be called multidrug-resistant Staphylococcus aureus or oxacillin-resistant Staphylococcus aureus (ORSA).

MRSA is, by definition, any strain of Staphylococcus aureus that has developed resistance to beta-lactam antibiotics which include the penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the cephalosporins.

MRSA is especially troublesome in hospitals and nursing homes where patients with open wounds, invasive devices and weakened immune systems are at greater risk of infection than the general public.